A hybrid electric vehicle has a propulsion system that consists of at least one electric motor that is utilized in some form with another power source. Most often the other power source is a gasoline or diesel engine.
Typically, the two power sources are configured in one of two ways, series and parallel. In a series hybrid the traction force to the vehicle's wheels is provided strictly by the electric motor. Electric energy is stored in a battery and is used to power the motor whenever demanded by the driver. The other power source, i.e. an engine, is used to maintain the level of energy stored in the battery at a level that is adequate to supply power to the electric motor as needed. In a series hybrid the engine is not used to provide direct motive force to the wheels based on driver demand. All of the energy from the engine is stored in the battery, where it is used by the electric motor to propel the vehicle.
In a parallel hybrid, both the engine and the motor can be directly coupled to the vehicle's wheels, so that both power sources can, independently, provide motive force for the vehicle. It should be noted that in a parallel hybrid, the engine is sometimes used to recharge the battery that supplies the motor, just as in a series hybrid.
In a specific configuration of a parallel hybrid, the motor only provides positive torque when it is used to boost the torque from the engine. This configuration requires far less energy from the battery and is often referred to as a Low Storage Requirement (LSR) architecture.
In the past, a vehicle system control algorithm has not existed for a hybrid electric vehicle. The system control algorithm is an important element used to coordinate the control of the power train components thereby managing energy in the vehicle and avoiding driveline disturbances.